Communication equipment, of which a radiotelephone is a convenient example, provides an appropriate setting for describing the need for the present invention.
Radiotelephones generally comprise audio, logic, and transceiver circuitry. The logic circuitry includes a processor that executes a sequence of predetermined operations, responsive to the predetermined frequency of a reference clock signal, to control the audio and the transceiver circuitry. A high frequency clock signal is usually needed for the processor to execute the operations at a rate required for typical radiotelephone operation.
High frequency clock signals are typically characterized by digital square wave pulses with sharp edges that contain significant spectral energy. The spectral energy produces harmonic frequency components, hereinafter called harmonics, at frequencies that are integral multiplies of the frequency of the clock signal. The harmonics may radiate from lines, carrying the clock signal, interconnecting the logic circuitry.
A common problem in the design of radiotelephones is that the radiated harmonics may interfere with information signals in a bandpass filter in the receiver portion of the transceiver circuitry. Radiated harmonics, falling within the frequency bandwidth of the receiver's bandpass filter, may have a power level that masks or distorts information signals of lesser power level resulting in poor information reception.
Traditional techniques implemented for reducing harmonic interference include shielding the receiver circuitry from the logic circuitry, placing the logic circuitry in a radiotelephone cavity separate from the receiver circuitry, and isolating currents in the receiver circuitry from the logic circuitry. Although these techniques provide adequate solutions, these techniques may require additional parts, consume valuable engineering development time and resources, and/or increase the size of the radiotelephone, all of which increase the cost of the radiotelephone. Furthermore, in the future, desirable smaller radiotelephone housings and higher speed signal processing will increase the likelihood and severity the harmonic interference.
For many situations, of which communication equipment is merely an example, the prior art has not produced an apparatus or method for reducing the power level of harmonics, within a predetermined frequency bandwidth, generated by a clock signal to solve the problem of harmonic interference while providing for the advantage of lower equipment cost.